1. Field of the Invention
The present invention relates to a piezoelectric bulk acoustic wave resonator and to its manufacturing method.
2. Discussion of the Related Art
Piezoelectric bulk acoustic wave (BAW) resonators are generally used to form filters or resonant elements. They may, in particular, be used in devices ensuring time or frequency reference functions (for example, temperature compensated X-Tal (crystal) oscillators, TCXOs).
A BAW resonator includes a layer of a piezoelectric material sandwiched between two metal electrodes. The electrodes are generally made of molybdenum, tungsten, aluminum, or platinum. When an electric excitation is applied between its two electrodes, the piezoelectric layer expands or shrinks widthwise. Mechanical vibrations are thus generated in the piezoelectric layer, such vibrations themselves creating an electric signal. The fundamental resonance of the system is observed when the mechanical vibrations and the induced electric field generate a constructive wave, thus optimizing the transfer power between the two vibrations.
To obtain BAW resonators with significant quality (Q) and electromagnetic coupling (Kt2) factors, it is generally provided to isolate these resonators from the substrate on which they are formed. This isolation confines the acoustic power in the resonator. To perform this isolation, two structures are known: the resonator may be suspended above a recess or it may be formed on an acoustic mirror (also called Bragg mirror). A disadvantage of a Bragg mirror is the multiplicity of the operations necessary to manufacture it.
FIG. 1 illustrates an example of a resonator suspended above a recess. This type of resonator, formed on a thin film, is currently called FBAR (film bulk acoustic resonator), TFBAR (thin film bulk acoustic resonator) or TFR (thin film resonator).
A membrane 12, for example, made of silicon nitride, is formed on a silicon substrate 10. Membrane 12 is separated from substrate 10 at its center by a recess 14, while its periphery is in contact with substrate 10. A stack 16 forming the resonant device is formed on the portion of membrane 12 distant from the substrate (above recess 14). Stack 16 comprises a first conductive layer 18 forming the first metal electrode of the resonator, a piezoelectric material region 20, and a second conductive layer 22 forming the second electrode of the resonator. It should be noted that, in this drawing and in the following ones, the contact recoveries on first and second electrodes 18 and 22 are not shown.
A disadvantage of a structure such as shown in FIG. 1 is the complexity of its manufacturing process. Indeed, to form suspended membrane 12, a sacrificial layer is formed under this membrane, after which this layer is removed. Other known variations enable forming a cavity under the resonant stack. One of them comprises forming an opening at the surface of the substrate, after which the resonant stack is formed on a planar intermediary layer extending over the substrate. This method has the disadvantage of requiring to form the intermediary layer on the substrate. Another variation comprises forming the resonator directly on the substrate and etching a portion thereof from its surface opposite to the resonator. However, this solution is typically possible only in the case of a relatively thin substrate and cannot be adapted to solid substrates.
Further, a disadvantage of resonant devices of finite dimensions such as that in FIG. 1 is that spurious acoustic modes form in the resonator, which alter the quality factor. A known method to characterize a resonant device is to draw its “dispersion curve”, which illustrates such spurious acoustic modes. Two types of curves can be distinguished, “type-I” dispersion curves in the case where the resonance frequency of the second harmonic of the transverse mode is lower than the resonance frequency of the longitudinal mode, and “type II” dispersion curves in the case where the resonance frequency of the second harmonic of the transverse mode is higher than the resonance frequency of the longitudinal mode.
According to the piezoelectric material used or to the resonator stack, the dispersion curves may be of type I or of type II. A known method to prevent the forming of spurious acoustic modes is to form a rigid frame at the periphery of the external surface of the resonant device. This technique is disclosed, in the case of dispersion curves of type I and II, in the publication entitled “Spurious mode suppression in BAW resonators”, by Robert Thalhammer et al., IEEE 2006, 1054-0117/06.
FIG. 2 illustrates a first example of such a resonant device, in the case where the piezoelectric material, or the entire resonant element, has a type-I dispersion curve. Such a piezoelectric material, for example, is zinc oxide.
The resonator comprises a stack 16 (first conductive layer 18, piezoelectric layer 20, and second conductive layer 22) formed on a support 24, for example, a Bragg mirror or an upper membrane. To avoid the generation of parasitic acoustic modes at the periphery of stack 16, a frame 26, for example made of silicon oxide, is formed at the surface of stack 16 over the entire periphery thereof. Frame 26 ensures the fitting between the active area of the resonator and its outer part.
FIG. 3 illustrates a second example of a resonant device in the case where the piezoelectric material or the entire resonant element has a type-II dispersion curve. Such a piezoelectric material, for example, is aluminum nitride.
The resonator comprises a stack 16 similar to that of FIG. 2 (first conductive layer 18, piezoelectric layer 20, and second conductive layer 22), formed on a support 24, for example a Bragg mirror or a suspended membrane. A frame 26, for example made of silicon oxide, enabling to attenuate the spurious acoustic modes at the periphery of stack 16, is formed at the periphery of the surface of stack 16. In this device, the thickness of second conductive layer 22 is decreased in a region 28 forming, in top view, a frame in internal contact with peripheral frame 26.
A disadvantage of the devices disclosed in FIGS. 2 and 3 is that their upper surfaces are strongly marked (topology) due to the presence of frame 26 at the surface of stack 16. Further, the forming of frame 26 needs at least one deposition step and one etch step, in addition to the resonant stack forming steps.